Spatial variation of sediment mineralization supports differential CO2 emissions from a tropical hydroelectric reservoir

Assessment of the impact of dams on aquatic food webs using stable isotopes: Current progress and future challenges

Dams have disrupted natural river systems worldwide and although population and community level effects on aquatic biota have been well documented, food web responses remain poorly understood and difficult to characterize. The application of stable isotope analysis (SIA) provides a means to assess the effect of dams on food webs. Here we review the effect of dams on aquatic food webs using SIA, aiming to detect knowledge gaps in the field of dam impacts on aquatic food webs and propose a conceptual framework to help formulate hypotheses about dam impacts on food webs guided by food web theory. Dams can affect aquatic food webs via two pathways: a bottom-up pathway with altered basal food sources and their transfer to consumers through changes in flow, nutrients, temperature and sediment, and a top-down pathway with consumer species composition altered mainly through habitat fragmentation and related physiochemical changes. Taking these mechanisms into consideration, the impact of dams on food web attributes derived from SIA was evaluated. These studies generally apply mixing models to determine how dams alter the dominant carbon sources supporting food webs, use δ15N to examine how dams alter food-chain length, or use Layman metrics of isotope variability to assess niche changes for invertebrate and fish assemblages. Most studies compare the patterns of SIA metrics spatially (e.g. upstream vs reservoir vs downstream of dams; regulated vs unregulated rivers) and temporally (before vs after dam construction), without explicit hypotheses and/or links to theoretical concepts of food webs. We propose several steps to make SIA studies of dam impacts more rigorous and enhance their potential for producing novel insights. Future studies should quantify the shape and strength of the effect of dams on SIA-measured food web response, be conducted at larger temporal and spatial scales (particularly along the river longitudinal continuum and the lateral connected ecosystems (e.g., floodplains)), and consider effects of dams on food web resilience and tipping points.

Sediment methane production within eutrophic reservoirs: The importance of sedimenting organic matter

While temperate reservoirs can be a significant source of atmospheric methane (CH₄), knowledge of the role they play in global emissions of the gas remains limited in line with extreme temporal and spatial variability noted both within and between reservoirs. There is also still no clear identification of the environmental factors influencing the emission of this gas to the atmosphere. This article presents the results of research into the influence of sedimenting matter on CH₄ emission from the surface of different zones of reservoirs. The research were conducted in 2018-2019 within Maziarnia and Nielisz Reservoirs - two artificial bodies of water of eutrophic status both located in SE Poland. Their diffusive CH₄ fluxes at the water-air interface were measured using the "static chamber" method, while sediment traps monitored the rate of accumulation of sedimenting matter in bottom sediments (U_S). The CH₄ fluxes noted at the reservoirs proved highly variable, both temporally and spatially, ranging from 0.02 to over 2500 mmol/m²·d. Determined accumulation indexes were in turn in the 13.61-618.49 g/m²·d range. Nevertheless, CH₄ flux was found to correlate significantly with sedimentation index (U_S), with highest observed values for both reservoirs noted in river zone, while the lowest characterise the lacustrine zone. On this basis, it was hypothesised that sedimentation index may prove a useful tool in estimating CH₄ emissions from reservoirs, with the reverse relationship also likely to apply. Furthermore, the key factor found to be responsible for the aforementioned temporal and spatial variations in CH₄ emissions is primary production, whose subsequent sedimentation supplies sediments with easily-degradable organic matter. The results presented here contribute to an understanding of environmental factors that may influence spatial variation in CH₄ production and can be useful to serve determinate of potential methods by which to reduce emissions this gas from managed systems such as reservoirs.

The interplay of nutrients, dissolved inorganic carbon and algae in determining macrophyte occurrences in rivers

Nitrogen and phosphorous concentrations are widely considered to drive macrophyte assemblages in rivers. However, Dissolved Inorganic Carbon (DIC) - available for plants as CO₂ and HCO₃^- - is also of major relevance. Based on literature, we present a conceptual model on the interaction between algae, macrophytes, DIC, pH, light, N, P and the surface water and sedimental compartment. Analysing two separate datasets (i) on river physico-chemistry and chlorophyll-a, and (ii) on river physico-chemistry and macrophytes we quantify three connections within this concept: (1) the correlation of chlorophyll-a versus pH, (2) the correlation of TP versus chlorophyll-a and (3) the occurrence of HCO₃-users and CO₂-only-users among macrophytes along the DIC gradient. Chlorophyll-a correlated positively with pH (R-squared = 77%, p < .001) due to increased carbon dioxide uptake of phytoplankton. Surface water TP did not linearly correlate with chlorophyll-a concentrations. Obligate and optionally submerged macrophyte species that utilise HCO₃^- were separated from CO₂-only-users by HCO₃^- concentrations, with an area under the curve (AUC) of 68% and 70% (both p < .001) between groups. Obligate and optionally submerged macrophyte assemblages only composed of HCO₃-users and those exclusively composed of CO₂-only-users showed an even stronger separation based on the HCO₃^- concentration, with both an AUC of 82% and 78% (both p < .001). Our results underline that DIC can greatly affect riverine macrophytes. However, absolute concentrations of HCO₃^- are less relevant, while the connection to pH is more important, reflecting CO₂ concentrations. River monitoring and management should consider the interaction between nutrients DIC, surface water and sedimental compartment as important factors affecting macrophyte occurrence, rather than solely focussing on surface water nutrients.

Regional and Long-Term Analyses of Stable Isotopes of Fish and Invertebrates Show Evidence of the Closure of a Pulp Mill and the Influence of Additional Stressors

A bleached kraft pulp mill discharging effluent to the Mattagami River in northern Ontario, Canada, closed after almost 90 yr of operation. During its operation, effluent from the mill influenced biota in the downstream areas. To assess shifts in the reliance of biota from mill-derived nutrients, the isotopic composition (δ¹³ C and δ¹⁵ N) of white sucker (Catostomus commersoni) muscle and whole mayflies (Hexagenia sp.) were compared before (1990s) and after the pulp mill's closure (2012-2014). To better understand other potential sources of spatial and temporal change, samples from 3 other tributaries in the basin with dams, ongoing pulp mill operations, sites receiving sewage, and at several reference sites were collected and compared. Irrespective of time period, biota collected at sites downstream of both dams and active pulp mills tended to have elevated δ¹³ C values, but variable changes in δ¹⁵ N (negligible in most cases) when compared with upstream samples. The isotopic composition of mayflies varied at reference sites over time, with decreasing values of δ¹³ C and δ¹⁵ N (mayflies only) with increasing depth, and there was evidence of lower δ¹³ C in fish after the pulp mill closure. Overall, these results suggest the importance of long-term, regional-scale measurements for documenting the effects of stressors on nutrient use by aquatic species. Environ Toxicol Chem 2020;39:1207-1218. © 2020 SETAC.

Effects of seasonality, trophic state and landscape properties on CO2 saturation in low-latitude lakes and reservoirs

The role of tropical lakes and reservoirs in the global carbon cycle has received increasing attention in the past decade, but our understanding of its variability is still limited. The metabolism of tropical systems may differ profoundly from temperate systems due to the higher temperatures and wider variations in precipitation. Here, we investigated the spatial and temporal patterns of the variability in the partial pressure of carbon dioxide (pCO₂) and its drivers in a set of 102 low-latitude lakes and reservoirs that encompass wide gradients of precipitation, productivity and landscape properties (lake area, perimeter-to-area ratio, catchment size, catchment area-to-lake area ratio, and types of catchment land use). We used multiple regressions and structural equation modeling (SEM) to determine the direct and indirect effects of the main in-lake variables and landscape properties on the water pCO₂ variance. We found that these systems were mostly supersaturated with CO₂ (92% spatially and 72% seasonally) regardless of their trophic status and landscape properties. The pCO₂ values (9-40,020 μatm) were within the range found in tropical ecosystems, and higher (p < 0.005) than pCO₂ values recorded from high-latitude ecosystems. Water volume had a negative effect on the trophic state (r = -0.63), which mediated a positive indirect effect on pCO₂ (r = 0.4), representing an important negative feedback in the context of climate change-driven reduction in precipitation. Our results demonstrated that precipitation drives the pCO₂ seasonal variability, with significantly higher pCO₂ during the rainy season (F = 16.67; p < 0.001), due to two potential main mechanisms: (1) phytoplankton dilution and (2) increasing inputs of terrestrial CO₂ from the catchment. We conclude that at low latitudes, precipitation is a major climatic driver of pCO₂ variability by influencing volume variations and linking lentic ecosystems to their catchments.

Redistribution of methane emission hot spots under drawdown conditions

In the context of reservoirs, sediment trapping, and aquatic greenhouse gas (GHG) production, knowledge about the distribution of hot and low spots is essential for improved measurement strategies. It is also a key to a precise assessment of the GHG emissions of each reservoir. Large numbers of reservoirs are used mainly for hydroelectric power generation and, hence, affected by strong changes in water level. Drawdown events may lead to significant changes in spatial sediment and organic carbon distribution and, consequently, strongly alter the GHG emission patterns of the water body. We combined hydroacoustic sediment classification, sediment magnitude detection, and ebullition flux assessment with in-situ pore water investigations and sediment coring to detect ebullition distribution patterns after strong reservoir drawdown. The research was conducted in the Capivari Reservoir in the southeast of Brazil, which was affected by up to 15 m of drawdown within the last 10 years. Results show severe changes in sediment accumulation and composition. The focusing of sediment divides the reservoir in extreme hot and low spots. Methane pore water concentrations are highly correlated with acoustic backscatter values (r² = 0.97) as well as with the organic carbon content (r² = 0.55) and allow for a precise detection of the newly created emission patterns. Highly productive sediment could be acoustically distinguished from non-productive areas. Only 23.6% of the reservoir surface produced 64% of the detected bubbles. An organic carbon content in the sediment of 2.4% was found to be a prerequisite for the formation of GHG emission hot spots. These findings may help to complement the still insufficient knowledge of methane ebullition fluxes from reservoirs with changing water levels.

Spatially Resolved Measurements of CO2 and CH4 Concentration and Gas-Exchange Velocity Highly Influence Carbon-Emission Estimates of Reservoirs

The magnitude of diffusive carbon dioxide (CO2) and methane (CH4) emission from man-made reservoirs is uncertain because the spatial variability generally is not well-represented. Here, we examine the spatial variability and its drivers for partial pressure, gas-exchange velocity (k), and diffusive flux of CO2 and CH4 in three tropical reservoirs using spatially resolved measurements of both gas concentrations and k. We observed high spatial variability in CO2 and CH4 concentrations and flux within all three reservoirs, with river inflow areas generally displaying elevated CH4 concentrations. Conversely, areas close to the dam are generally characterized by low concentrations and are therefore not likely to be representative for the whole system. A large share (44-83%) of the within-reservoir variability of gas concentration was explained by dissolved oxygen, pH, chlorophyll, water depth, and within-reservoir location. High spatial variability in k was observed, and kCH4 was persistently higher (on average, 2.5 times more) than kCO2. Not accounting for the within-reservoir variability in concentrations and k may lead to up to 80% underestimation of whole-system diffusive emission of CO2 and CH4. Our findings provide valuable information on how to develop field-sampling strategies to reliably capture the spatial heterogeneity of diffusive carbon fluxes from reservoirs.

Phosphorus availability as a primary control of dissolved organic carbon biodegradation in the tributaries of the Yangtze River in the Three Gorges Reservoir Region

Biodegradability of dissolved organic carbon (DOC) represents a critical component of the riverine C cycle. Current knowledge of DOC biodegradation in rivers is limited, especially in the subtropical regions. Here, we collected 66 water samples from 63 tributaries of the Yangtze River in the Three Gorges Reservoir Region, China, and subsequently examined the biodegradability of DOC and its controlling factors. We found that DOC biodegradation was quite spatially variable within the river networks and ranged from 15.8% to 35.2%, with a mean of 24.5±8.0%. The biodegradability of DOC was positively correlated with the initial dissolved total phosphorus (P) concentration, but was not significantly correlated with the initial DOC and dissolved total nitrogen (N) concentrations. In addition, DOC biodegradation was negatively correlated with the initial C:P and N:P ratios, and exhibited no significant relationship with the initial C:N ratio in these rivers. Our findings suggest that DOC biodegradation is limited by P availability in the subtropical rivers, and also imply that P enrichment induced by anthropogenic activities would enhance the biodegradability of DOC and decrease the spatial heterogeneity of DOC biodegradation in the subtropical river networks.

High Primary Production Contrasts with Intense Carbon Emission in a Eutrophic Tropical Reservoir

Recent studies from temperate lakes indicate that eutrophic systems tend to emit less carbon dioxide (CO₂) and bury more organic carbon (OC) than oligotrophic ones, rendering them CO₂ sinks in some cases. However, the scarcity of data from tropical systems is critical for a complete understanding of the interplay between eutrophication and aquatic carbon (C) fluxes in warm waters. We test the hypothesis that a warm eutrophic system is a source of both CO₂ and CH₄ to the atmosphere, and that atmospheric emissions are larger than the burial of OC in sediments. This hypothesis was based on the following assumptions: (i) OC mineralization rates are high in warm water systems, so that water column CO₂ production overrides the high C uptake by primary producers, and (ii) increasing trophic status creates favorable conditions for CH₄ production. We measured water-air and sediment-water CO₂ fluxes, CH₄ diffusion, ebullition and oxidation, net ecosystem production (NEP) and sediment OC burial during the dry season in a eutrophic reservoir in the semiarid northeastern Brazil. The reservoir was stratified during daytime and mixed during nighttime. In spite of the high rates of primary production (4858 ± 934 mg C m^-2 d^-1), net heterotrophy was prevalent due to high ecosystem respiration (5209 ± 992 mg C m^-2 d^-1). Consequently, the reservoir was a source of atmospheric CO₂ (518 ± 182 mg C m^-2 d^-1). In addition, the reservoir was a source of ebullitive (17 ± 10 mg C m^-2 d^-1) and diffusive CH₄ (11 ± 6 mg C m^-2 d^-1). OC sedimentation was high (1162 mg C m^-2 d^-1), but our results suggest that the majority of it is mineralized to CO₂ (722 ± 182 mg C m^-2 d^-1) rather than buried as OC (440 mg C m^-2 d^-1). Although temporally resolved data would render our findings more conclusive, our results suggest that despite being a primary production and OC burial hotspot, the tropical eutrophic system studied here was a stronger CO₂ and CH₄ source than a C sink, mainly because of high rates of OC mineralization in the water column and sediments.